In a first prior art approach, an independent block partition and, hence, independent spatial predictors are selected for each color component. For example, the 16×16 block partition with one spatial prediction mode may be selected for the red channel, an 8×8 block partition with four spatial prediction modes may be selected for the green channel, and a 4×4 block partition with sixteen spatial prediction modes may be selected for the blue channel.
Conversely, in a second prior art approach, a common block partition is used for all three channels, which is consistent with a definition of macroblock type in a third prior art approach. In addition, a common set of spatial predictors are used for all three channels. Following the above example, in the case of the second prior art approach, the mode selector might have selected an 8×8 block partition as the macroblock type, and each channel would use exactly the same four spatial prediction modes by minimizing the predefined cost function. Obviously, the common mode approach greatly reduces the decoder complexity compared with the independent mode where three spatial prediction modes instead of a single spatial prediction mode have to be decoded for every coding block. In the meantime, since using a common prediction mode instead of three separate modes reduces the total number of bits to encode the spatial prediction information, the common mode solution results in better overall compression performance compared with the independent mode, especially for the mid and low bitrate range. A typical prior art implementation of the common mode method proceeds by examining each channel in turn (i.e., serially) to determine the best spatial predictors. This is a disadvantage when compared to the implementation of the independent channel method, since in that case the optimum spatial predictor for each channel can be derived in parallel in a straightforward way, thus potentially increasing the speed at which the video data is encoded.